Computers store data on magnetic discs, both of the "floppy" and "fixed" type, as well as on magnetic tape. The operation of a disc drive can be explained with reference to FIGS. 1A-1C.
A rotating disc 1 contains data arranged in concentric tracks 2. To read data, a movable read/write head 5 in FIG. 1B is positioned above the particular track 2 containing the data of interest.
The data bits can be viewed as tiny magnets 7 in FIG. 1C. When a moving bit-magnet passes beneath the head 5, it excites a coil 9 within the head 5, inducing an electrical signal in the coil. The induced signal is small, of the order of a few millivolts. Further, the size of the signal changes as radial distance of the head changes. That is, when the head 5 is positioned near the center of rotation, at a small radius, the relative speed of rotation between the head 5 and the disc 1 is small. The small speed produces a small signal.
Conversely, when the head 5 is positioned far away from the center, the speed is large, and the signal produced by the coil 9 is large.
(In more complex terms, the signal produced by the coil depends on d.PHI./dt, which is the first time derivative of flux density, .PHI.. The lines 10 in FIG. 1C indicate flux lines.)
The signal must be amplified. In the amplifier, Automatic Gain Control (AGC) circuits are used to compensate for the change in signal magnitude. When the signal is small, the gain of the amplifier is made large. When the signal is large, the gain is less.
The amplifiers in common use today are differential amplifiers which use bipolar junction transistors. As the technology of disc drives advances, such amplifiers will become unfeasible to use, because of their relatively large size and large power consumption. Further, using CMOS technologies allows analog circuitry to be integrated with complex digital circuitry on a single chip, in order to obtain higher levels of integration.